In U.S. Pat. No. 6,762,730, entitled CROSSED BOWTIE SLOT ANTENNA, by one of us, filed May 18, 2000, and incorporated by reference with regard to radiator layout and emission properties (hereinafter the '730 patent), a single bowtie slot radiator is disclosed as having a pattern of signal intensity referred to in the art as “peanut” shaped. This term refers to the radiator's characteristic emission in substantially equal nodes (of opposite phase) perpendicular to the faces of the bowtie slot. This emission pattern has nulls, or locations of relatively low signal strength, generally in the plane of the bowtie slot radiator. Where the radiator is oriented as shown in the '730 patent and herein, the polarity of propagation is horizontal, a property required of television broadcast signals by regulation.
As further noted in the '730 patent, when two such radiators are joined at right angles to form a single bay of a crossed bowtie slot antenna, and when the phase angle of the signals applied to the two slots formed thereby is properly chosen, then azimuth uniformity of the emission pattern approaches that of a simple dipole in free space. Each of the radiators has approximately the same nodes and nulls as when standing alone, with little interference between them. Since the nodes of each radiator lie in the plane of the other, the combined bay has four nodes, and the nulls of the combination fall midway between the planes of the two radiators. Stacking multiple bays vertically and energizing the bays with suitable signal strength and phase can increase gain, narrowing beamwidth in the vertical plane. Reinforcement increases reception range parallel to the plane of the earth, while cancellation decreases signal levels directed upward and downward.
An antenna based on this design may perform well, not just at a particular frequency for which the dimensions are optimized, but, by virtue of the features of the '730 patent, including the bowtie slot shape, over a broad frequency range. Indeed, when energized with multiple television channel signals, each having a characteristic bandwidth on the order of 6 MHz and separated in frequency to include guard bands and excluded channels, so that the excitation is distributed over some tens of megahertz, an antenna according to the '730 patent can meet demanding performance criteria.
Nonetheless, it is noteworthy that azimuth uniformity of at least some styles of crossed bowtie slot antennas tends to decrease toward the upper limit of the antennas' working ranges. This decrease has been shown to take the form of reduction in radiative intensity at the nulls noted above, that is, at angles roughly intermediate between signal nodes, with the nulls becoming more prominent with increased frequency. It would be desirable for some broadband applications to provide still greater azimuth uniformity over frequency.
Accordingly, there is a need in the art for increasing crossed bowtie slot antenna signal strength uniformity with azimuth over frequency.